うっせぇわ

その可もなく不可もないメロディー: Literally means "That melody without positives, without negatives". You interpret it as just average, but I tried to follow the structure closely with "strengthless and flawless".

だったら言葉の銃口を...その頭に突きつけて撃てば: Meaning is kinda blurred because of the enjambement. The "muzzle" I use is like the muzzle of a gun (to which the "words" are likened). So the full sentence is like "If that's the case, I'll thrust the muzzle of your words into your face and shoot."

皆がつまみ易いように串外しなさい: I think this is like making someone remove burning hot skewers off a grill so that others can eat them without getting burnt lol.

会計や注文は先陣を切る: I interpreted this as meaning that a group's bills and orders will jump to the top of the person's to-do list

二番煎じ言い換えのパロディ: Literally, parody of putting a rehash in other words. Since a rehash itself is putting something in other words, I change other into new.